Conventional CMOS and CCD digital image sensors use a photodiode, or a MOS photogate, as the photosensing element. In its native state, a photosensing element captures a light signal only as the presence or absence of light, i.e., only as white or black, respectively. In order to capture a color image, color filters are placed on top of each photo sensing element. Usually, red, green, and blue (RGB) color filters are arranged in a Bayer filter pattern. A typical Bayer filter pattern includes alternate filter placement to capture individual RGB pixels, as shown in Table 1.
TABLE 1RGRGGBGBRGRGGBGB
Bayer pattern capture is described in U.S. Pat. No. 3,971,065, to Bayer, granted Jul. 20, 1976, for Color Imaging Array, which provides a sensing array for color imaging, having individual luminance and chrominance sensitive elements, which are intermixed such that each type of element, i.e., according to sensitivity characteristics, occurs in a repeated pattern, with luminance elements dominating the array. Preferably, luminance elements occur at every other element position to provide a relatively high frequency sampling pattern which is uniform in two perpendicular directions, e.g., horizontal and vertical. The chrominance patterns are interlaced therewith and fill the remaining element positions to provide relatively lower frequencies of sampling. A mosaic of selectively transmissive filters is superposed in registration with a solid state imaging array having a broad range of light sensitivity, the distribution of filter types in the mosaic being in accordance with the above-described patterns.
Image-processing, following image capture by a Bayer pattern sensor, includes interpolation, during which missing data is estimated from data gathered by neighboring pixels. This results in a fuzziness in an image captured by a Bayer pattern sensor, as the image is less sharp than it otherwise would be because of under sampling. Such images also exhibit color aliasing artifacts because of lateral mis-alignment of the color filters.
A stacked RGB photodiode can directly measure red, green, and blue at the same location, by stacking three photodiodes on top of one another. This technology increases the sampling density, improves sharpness, and eliminates the color aliasing artifact. More importantly, this technology does not require color filters. An example of three-color visible-light vertical pixel sensors group is disclosed in U.S. Pat. No. 5,965,875 to Merrill, granted Oct. 12, 1999, for Color separation in an active pixel cell imaging array using a triple-well structure, which describes a structure using a triple-well CMOS process wherein the blue, green, and red sensitive PN junctions are disposed at different depths beneath the surface of the semiconductor substrate upon which the imager is fabricated, as shown in FIG. 1, as taken from '875.
Findlater et al., A CMOS image sensor with a double junction active pixel, IEEE Trans. Electron Devices, Vol. 50, No. 1, pp 32-42 (2003), disclose an active pixel sensor that incorporates a double-junction photodiode in conjunction with an organic filter overlay, wherein each double-junction photodiode includes top and bottom p-type layers with an n-type layer between them. The n-type layer forms the cathode of a first photodiode, the bottom p-type layer forms the anode of a second photodiode, the first photodiode is coupled to a first readout circuit, and the second photodiode is coupled to a second readout circuit. A filter mosaic overlays the sensor array.
U.S. Pat. No. 6,727,521 to Merrill, granted Apr. 27, 2004, for Vertical color filter detector group and array, describes the formation of the three stacked photodiodes comprising at least six layers of alternating p-type and n-typed doped regions, as shown in FIG. 2, taken from '521. PN junctions between the layers operate as photodiodes with spectral sensitivities that depend on the absorption depth versus wavelength of light in the semiconductor. Alternate layers are detector layers to collect photo-generated carriers, while the intervening layers are reference layers and are connected in common to a reference potential referred to as ground. In an embodiment where the detector layers are n-type layers, each detector group includes a blue photodetector n-type layer at the surface of the semiconductor, a green photodetector n-type layer deeper in the semiconductor, and a red photodetector n-type layer deepest in the semiconductor. Three sets of active pixel sensor circuitry are coupled to the three detector layers.
The sensor group of '521, having three stacked photodiodes with six layers of alternating p-type and n-typed doped regions, employs two silicon epitaxial processes and the three set of active pixel sensor circuitry are fabricated on the top surface of the second epitaxial layer. The isolation between the pixel transistors and the isolation between the pixel transistor and photodiode limit the area of the unit image cell. Furthermore, the processes to make an isolated, low resistance electrical contact to the red photodetector n-type layer deep in the silicon substrate are complicated and further limit the area of the unit image cell.